flybrain/docs/readout.md

Readout

PopulationDecoder (packages/brain/src/readout/decoder.ts) turns per-role population rates into a set of active output channel names. Nothing about the downstream device enters the module: channels are named by the caller and the only input is the role -> rate record the network produces. Device specifics live in presets under src/readout/presets/.

Score

Every decision uses one normalized score:

score(channel) = (rate[role] + 1) / (baseline[role] + 1)

baseline is recorded once by calibrate(rates), which stores the current rate of every channel's role. The +1 on both sides bounds the ratio when rates are near zero. A score of 1 means "at rest", above 1 means "above its calibration rate". Source: readout/decoder.ts, decode(); original wording in fly-plays-pokemon/docs/ratchet-sprint.md, "Controller".

An uncalibrated decoder returns an empty array from decode() and changes no state.

Exclusive group

Up to two optional ExclusiveGroups, each with its own channels, winner, fatigue and decision clock: exclusive, the device's directions, and macros, the macro buttons (see "Macro group" below). At most one of a group's channels is active at a time, and that group's winner is re-decided only when nowMs >= nextDecision. On a decision:

0. (Before the decision, on every decode: a blocked-direction report raises that channel's fatigue. See below.)
1. Divide each group score by 1 + fatigue[channel].
2. Take the argmax over the candidates — every channel of the group, except in the macro group, where the caller names them. Ties keep the earlier channel, because only a strictly greater score displaces the running best, and channel order is the insertion order of channels.
3. If there is a current winner, it is still a candidate, and score[best] < score[current] * hysteresis, keep the current winner. At hysteresis = 1.05 a challenger must lead by 5% to take over; at 1.00 there is no commitment bonus and the argmax always wins.
4. Clear every group channel's hold, then set the winner's hold to nowMs + holdMs.
5. Add fatigueGain to the winner's fatigue, capped at 1; multiply every other channel's fatigue by fatigueDecay.
6. Set nextDecision = nowMs + decisionMs.

Fatigue is bounded habituation: it stops a small persistent rate bias from holding one channel forever. It is motor history, not reward learning and not a navigation policy.

The cap matters and is worth stating plainly: fatigue lives in [0, 1], so habituation can discount an incumbent's score by at most a factor of two. A channel whose raw score leads the field by more than 2 x hysteresis is a permanent winner however long it holds, and a field whose scores are all within hysteresis of each other -- which is what a settled network on the live stream actually produces -- can only ever be reordered by fatigue, never by score. fatigueGain is therefore not a tie-breaker; it is the rotation period.

Blocked-direction cooldown

decode(rates, nowMs, boot, blocked, bound) takes one input that is not a rate: the name of an exclusive channel the caller has observed producing no effect. (bound is the macro group's, and is described under "Macro group".) Before anything else in a decode, a report sets that channel's fatigue to blockedFatigue:

fatigue[blocked] = min(1, max(blockedFatigue, fatigue[blocked]))

max, not +=: the caller reports the same wall on every frame of a hold, and one wall is one penalty however many frames observe it. A report never lowers a fatigue the channel has already earned, and it is bounded by the same cap as fatigueGain.

That is the whole rule. It adds no state — fatigue is already in DecoderState, so the decoder version and the compatibility string are untouched — and it does nothing at all when blockedFatigue is 0, when blocked is null, or when blocked names a channel outside the group (a pulse channel, or a typo). decode without the argument is byte-identical to the three-argument decode that came before it. A blocked report names a channel, so it reaches whichever group holds that name: the macro group has the same field and the same rule, for the caller that one day reports a macro that moved nothing.

Why fatigue and not a lockout. Habituation is the mechanism the group already has for "this channel has had its turn", and the whole point is that a blocked direction should lose the next decision rather than be forbidden. A wall the fly is facing may stop being a wall — it is standing on a warp tile, a sprite moved, it turned a corner — and a channel whose fatigue decays back over the following few decisions recovers by itself. A hard lockout would need a new timestamp in the checkpoint and a rule for when to lift it.

blockedMs is the caller's number, not this module's. The decoder has no clock of its own beyond nowMs and no idea what "movement" means, so it cannot decide when a direction has failed. It publishes the window in the preset, next to the hold it is measured against, and the caller applies it. In flysim that caller is the sim loop: it keeps the adapter's last reported position (GameAdapter::location, the area and the tile) and the channel the group is holding, and reports that channel once both have been unchanged for blockedMs. Restarting the window on a new winner is what stops one stuck hold blaming the direction that follows it, which has not had a hold to move in yet. A location of null -- a battle, a script, a map transition -- is no information rather than "still", so the rule cannot fire while the fly has no control anyway.

What it is not. It says "that button did nothing". It does not say where the fly is, where it should go, what the room looks like, or which button to press instead; the winner is still the argmax of the same normalized scores over the same rates. It is the same kind of fact as the frame the retina already sees, arriving through a much narrower channel, and it is disclosed on the honesty panel with the rest of the readout.

Where the location comes from on the session framework (2026-09-23). When the live fly runs on the session framework (the operator's port decision of 2026-09-23), the sim loop that owned the position is split: the task reads the location, the agent owns the decoder. The location then reaches the decoder as a declared field of its decision context, gameboy-readout-context-v1 {boot, bound, location} (legacy Game Boy composition section 5). The rule is unchanged: the location only restarts the blocked window, null is still no information, the held channel and the window stay the readout's own state, and the blocked channel is still computed here, never handed in. The profile allowlists the field, and the task cannot put anything else in it.

Macro group (2026-09-16)

DecoderConfig.macros is a second ExclusiveGroup with the same fields and the same decision rules as the direction group above, decided after it on its own clock. Its channels are the macro types of docs/design/macros.md sections 12, 13 and 14: 31 of them, one per type, and a channel's name is its rate role (macro_go_item, macro_move_1, …), because the population is the button and a second name for it would be a second thing to keep in step. There are no pulses in the group.

Twenty-two when section 11 shipped. Section 13 added GO SHOP, GO HEAL, BUY ANTIDOTE, BUY REPEL and HEAL; section 14 replaced ATTACK with MOVE 1..MOVE 4, added THROW BALL and renamed GO STAIRS to GO WARP. Nothing about the rules above changed with either.

The one thing that differs is who may win. decode(rates, nowMs, boot, blocked, bound) takes the channels the caller has bound — the scene's own buttons — and the argmax runs over those alone:

• an unbound channel is masked out of the decision entirely, not penalised. That is the opposite end from the blocked-direction cooldown, which habituates a channel that can still win; an unbound channel is not on the pad at all, so it cannot.
• a masked channel's fatigue still relaxes by fatigueDecay, exactly as a loser's does. A type the scene took away is resting, not accumulating.
• the winner is therefore always one of the bound set, and the incumbent's commitment bonus applies only while it is still bound: a macro the scene has taken away cannot hold its own seat.
• bound of null lets the whole group compete, which is what the direction group always does. An empty bound is a scene that binds nothing: no winner, no hold, no fatigue, and nextDecision does not move, so the frame a button appears on is a frame that can press it.

macroChannelNames is the group's channels in config order and macroWinner is its current winner, which is the macro the game layer starts. In raw mode there is no group to consult: the preset is built with no macro roles, which is also every non-Pokémon adapter, so the decoder has no macro channels, no macro state and nothing to mask.

The 31 populations are a dataset artifact, not a decoder one: tools/build_flywire.py shares the 96 mushroom body output neurons and the 110 brain motor neurons — 206 neurons, six or seven each — round-robin over the types in the contract's own order, and emits them into data/fafb-v783/circuit-roles.json with the checksums in tools/artifact-checksums.txt. Neuron ids, edges, kernel and every pre-existing anatomical role are byte-identical — but 206 neurons dealt thirty-one ways is not twenty-two ways plus nine, so every macro population is re-dealt whenever a type is added, exactly as it was when section 11's amendment went from six slot channels to twenty-two type channels. Rates are restored by role name and a new or renamed role starts at zero, so a checkpoint written before the change loads; what a run loses across it is the mushroom body's learned preference for particular macros. The macro roles are deliberately outside the dataset fingerprint — fingerprintDataset hashes the metadata with the macro_* roles removed, in both twins — so flysim --print-compatibility is unchanged and a checkpoint written before the populations existed still loads, with their rates starting at zero.

Pulse channels

Any number of independent PulseChannel entries, evaluated every decode() call in config order. A channel fires when score > threshold and nowMs >= nextAllowed. Firing sets heldUntil = nowMs + holdMs and nextAllowed = nowMs + cooldownMs.

• boot: an alternate { cooldownMs, threshold } used while decode(rates, nowMs, boot) is called with boot true, which is the default. holdMs is never overridden. A channel without a boot variant behaves identically in and out of boot.
• throttleGroup: when a channel with a throttle group fires, every channel in that group gets the same nextAllowed, so the group shares one cooldown.

Active channels

active = channels where nowMs < heldUntil[channel]

returned as direction channels first in config order, then pulses in config order, then the macro group's channels. channelNames exposes that same order. The macro channels go last so that every index and every record order a consumer already reads is untouched by their arrival, and they are not joypad names, so toButtonMask ignores them the way it ignores any unknown name.

clearHolds(nowMs) drops every hold, forgets both groups' winners, zeroes all fatigue including the macro group's, sets both nextDecision clocks to nowMs so the next decode() re-decides immediately, and locks every channel out until nowMs + clearLockoutMs. Use it when the environment changes under the readout, for example after a state restore, so a stale hold cannot leak into the new context.

Game Boy preset

gameboyDecoderConfig(macroRoles) (src/readout/presets/gameboy.ts) is the only device-specific module in the library. The D-pad is the exclusive group; A and B are fast pulses; Start and Select are rare pulses sharing one throttle, with a permissive boot variant so title screens and new-game menus still work; macroRoles, the macro_<type> roles of the running adapter, becomes the macro group, and an empty list (the default) builds no macro group at all. Every value except the direction hold, the decision period, the fatigue gain, the hysteresis and the blocked-direction cooldown is the prototype's fixed motor decoder unchanged.

Exclusive group, decisionMs 800, holdMs 800, hysteresis 1.05, fatigueGain 0.08, fatigueDecay 0.8, blockedFatigue 0.35, blockedMs 800. clearLockoutMs is 480.

Macro group (2026-09-16): the same seven numbers to the digit, over one channel per macro_<type> role. They are the direction group's because a macro is a button, and because the measurement that chose them (below) measured how long a commitment should last, not what it was to. No macro channel has a bit: the game layer runs the winning macro, and the macro's own presses reach the emulator through the same button register the fly's raw buttons use.

Channel	Kind	Role	Bit	Hold	Cooldown	Threshold	Boot	Throttle
up	exclusive	command_0	0x01	800 ms
down	exclusive	command_1	0x02	800 ms
left	exclusive	command_2	0x04	800 ms
right	exclusive	command_3	0x08	800 ms
a	pulse	command_4	0x10	85 ms	480 ms	1
b	pulse	command_5	0x20	85 ms	480 ms	1
start	pulse	command_6	0x40	55 ms	30000 ms	1.35	2500 ms, 1	system
select	pulse	command_7	0x80	55 ms	30000 ms	1.35	2500 ms, 1	system

The bit column is GAMEBOY_BUTTON_BITS, the standard joypad mask. toButtonMask(active) packs active channel names into that mask, ignoring unknown names; fromButtonMask(mask) unpacks in bits key order. GAMEBOY_BUTTONS is the eight names as a const tuple.

In the prototype, boot mode was the reward adapter's active-game gate, so title and new-game operation kept the permissive Start/Select threshold while live play used the strict one (fly-plays-pokemon/docs/ratchet-sprint.md, "Controller").

Changed 2026-09-16: the direction hold, from 400 ms to 800 ms

decisionMs and holdMs went from 400 to 800 and fatigueGain from 0.08 to 0.04. docs/design/room-escape.md section 1 asked for it and infra/docs/room-escape.md holds the measurement that chose the value: one overworld step in Pokémon Red is about 270 ms, so a 400 ms commitment was one to two steps and the walker jittered in place instead of crossing a room. Over sixty seeded random-walker runs of ten brain minutes each, a fly escapes Red's bedroom in a mean 27.0 brain seconds at 800/0.04 against 52.4 at 400/0.08, and the real brain leaves it in all five of five confirmation runs.

Nothing about the decoder itself changed, and readout state is not part of the neural checkpoint, so DecoderState's version 4, the kernel version strings and the compatibility string are all untouched: an existing checkpoint loads and simply decodes on the new timings. What did have to move is the golden agent scenario (services/flysim/golden/agent.flygold), whose per-frame button masks are the preset's own output, and the two MotorDecoder oracles in packages/brain/tests, which now spell the prototype's 400 ms and 0.08 out for themselves — the oracle proves the decoder's logic, and the preset's numbers are pinned by their own tests.

Changed 2026-09-16 (v0.1.1): hysteresis to 1.05, fatigue gain back to 0.08, the cooldown on

hysteresis went from 1.15 to 1.05, fatigueGain from 0.04 back to the prototype's 0.08, and the blocked-direction cooldown was switched on at blockedFatigue 0.35, blockedMs 800. decisionMs and holdMs stayed at 800. docs/design/room-escape.md section 3 asked for it and infra/docs/room-escape.md holds the measurement that chose the values: this time not a random walker from a cold boot but the live release fly, run forward from its own generation-525 checkpoint, because the bug the change fixes is invisible to both a fresh boot and a walker.

Out of Red's house within ten brain minutes, fifteen runs per row from that checkpoint:

holdMs	fatigueGain	hysteresis	blockedFatigue	out of the house	inside 5 min	median
800	0.04	1.15	off	1/15	0/15	573.5 s
800	0.08	1.15	off	11/15	4/15	314.6 s
800	0.08	1.15	0.35	7/15	4/15	282.7 s
800	0.08	1.00	off	8/15	3/15	330.2 s
600	0.08	1.05	off	8/15	4/15	276.7 s
800	0.08	1.05	off	13/15	6/15	312.3 s
800	0.08	1.05	0.35	14/15	13/15	130.3 s

Three mechanisms, and they only make sense together.

• The hysteresis was the lock. On a settled network the four direction scores sit inside a 9% spread, so a challenger can never lead by 15% and the winner can only ever be changed by fatigue. 1.05 is a margin the spread can actually overcome. 1.00, no commitment bonus at all, is worse than either (8/15): the hold is a commitment device and removing the last of the bonus wastes it.
• The fatigue gain sets the length of a run of holds, not of one hold. At 0.04 one direction won 4.1 consecutive holds — 3.3 s, about twelve tiles at 268 ms a step, in a room eight tiles wide — and 62 to 64% of holds moved the fly nowhere at all. 0.08 halves that to 2.5 holds.
• The cooldown only pays off once the hysteresis is low. At 1.15 it made things worse, because a fly pushed off walls faster orbits the perimeter faster and Red's staircase is on the perimeter. At 1.05 the fly turns inward instead, and the cooldown stops each crossing being spent on a wall: time on the two staircase tiles falls from 16,268 frames to 3,642 over the fifteen runs.

Confirmed on a fresh boot with the real brain, five runs each: 5/5 out of the house from the ground floor (median 53.2 s, 4/5 inside five minutes) and 5/5 from the bedroom (median 83.8 s, 4/5 inside five minutes), leaving the bedroom itself in a median 20.3 s against v0.1.0's 30.3 s. The random walker, which cannot exercise the cooldown, still leaves both rooms 20/20 at these timings and reaches the outside from the bedroom in a median 122.2 s against 162.1 s at v0.1.0's values.

Nothing about the decoder's state changed, and readout state is not part of the neural checkpoint, so DecoderState's version 4, the kernel version strings and the compatibility string are all untouched: the live fly's checkpoint loads and simply decodes on the new numbers. What did have to move is the golden agent scenario (services/flysim/golden/agent.flygold), whose per-frame button masks are the preset's own output, and both legacyGameboyConfig twins, which now spell out the prototype's 1.15 and switch the cooldown off — the MotorDecoder oracles prove the decoder's logic against the prototype's numbers, and those are no longer the preset's.

Platformer preset

platformerDecoderConfig() (src/readout/presets/platformer.ts, and its Rust twin flybrain-core::decoder::platformer) is the same eight channels, bits and roles with different timings. Every value is argued from the Super Mario Land disassembly in docs/design/platformer.md §4; the summary is that a side-scroller needs commitment without gaps, a real jump, a held B, and no pausing.

Exclusive group, decisionMs 250, holdMs 250, hysteresis 1.25, fatigueGain 0.04, fatigueDecay 0.85. clearLockoutMs is 300.

Channel	Kind	Role	Bit	Hold	Cooldown	Threshold	Boot	Throttle
right	exclusive	command_3	0x08	250 ms
left	exclusive	command_2	0x04	250 ms
down	exclusive	command_1	0x02	250 ms
up	exclusive	command_0	0x01	250 ms
a	pulse	command_4	0x10	300 ms	420 ms	1.10
b	pulse	command_5	0x20	600 ms	200 ms	1.05
start	pulse	command_6	0x40	55 ms	600000 ms	2.0	2500 ms, 1	system
select	pulse	command_7	0x80	55 ms	600000 ms	2.0	2500 ms, 1	system

Three consequences worth stating plainly, because they are not visible in the numbers:

• holdMs === decisionMs means a winning direction never gaps: the re-decision lands exactly as the hold expires. Momentum in Super Mario Land decays whenever no direction is held, and momentum is what clears gaps.
• b's cooldown is shorter than its hold, so a channel whose score stays above threshold refires the instant the hold expires. That is a sustained hold built out of a pulse channel, with no decoder change, and it releases within 600 ms of the score dropping.
• Both system channels share one throttle group, so Start and Select can never be held in the same frame. Super Mario Land soft resets when A, B, Select and Start are held together, so the reset combination is structurally unreachable rather than merely unlikely.

right is first in channels, so the tie-break above favours rightward travel. That is a fixed prior, disclosed on the honesty panel rather than hidden.

The two presets share GAMEBOY_BUTTONS, GAMEBOY_BUTTON_BITS, toButtonMask and fromButtonMask; presets/platformer.ts re-exports them. The adapter picks a preset by returning a DecoderPresetId (flybrain-gb), so the sim loop never names a game.

State and legacy import

exportState(): DecoderState writes version: 4, calibrated, baseline, heldUntil, nextAllowed, nextDecision, current and fatigue, plus the macro group's macroNextDecision, macroCurrent and macroFatigue (2026-09-16).

The version stays 4. Those three fields are additive and optional on import: a checkpoint written before the macro group existed carries none of them, and the group then starts rested — no winner, every fatigue zero, its clock at 0 — which is the same rule the network's new rates follow. Macro channels are likewise exempt from "every configured channel is present in heldUntil and nextAllowed", for the same reason and no other: a present macro field is validated exactly as its direction twin is.

importState() accepts version 4 and the prototype's MotorDecoder checkpoints, versions undefined, 2 and 3, whose channel names match the configuration:

Version	Mapping
4	fields as written
3	nextDirectionDecision to nextDecision, direction to current, fatigue required
2 and undefined	same renames, and fatigue starts at zero because these predate habituation

Any other version number is rejected with Invalid decoder version. Every field is validated before anything is written, so a rejected checkpoint leaves the decoder untouched: calibrated must be a boolean, nextDecision finite, every value in baseline, heldUntil and nextAllowed finite, every direction and pulse channel present in heldUntil and nextAllowed, every fatigue value finite in [0, 1] for versions 3 and 4, and current either null or a channel of the exclusive group. macroNextDecision, when present, must be finite; every present macroFatigue value finite in [0, 1]; macroCurrent either null or a channel of the macro group.

Channels added after a checkpoint was written (2026-09-17)

A checkpoint carries baseline for the roles the decoder had when it was written. A group added since — the macro channels are the case this was found on — has no entry, and an absent baseline reads as zero, so the score

(rate + 1) / (baseline + 1)

becomes rate + 1: the channel competes on its raw rate against channels normalized to about

1. Population rates are not comparable that way. Live on 2026-09-17, forty-eight minutes in Oak's lab with TALK never chosen once, the macro populations read

role	rate
macro_next	145 Hz
macro_npc	107 Hz
macro_objective	65 Hz
macro_frontier	54 Hz
macro_talk	41 Hz
macro_run	27 Hz

macro_talk at 41 cannot beat macro_frontier at 54 in its one facing window, whatever the fly does. Nothing was wrong with the populations: the decoder had never been told what rest looks like for them, because calibrate() runs once at warm-up and the live state was restored from a checkpoint that predates the roles.

The rule. On import, the roles the checkpoint carried no baseline for are recorded as pending. The first decode after the restore calibrates them from its own rates, before any score is computed, so every such channel scores exactly 1.0 on that decision — at rest, which is the honest reading of a channel nobody has measured — and from the next decision on the group decides on merit against a baseline of its own.

• One sample, not a window. That is what warm-up does (NeuralAgent.warmup: settle for warmupMs, then calibrate on one snapshot of the settled rates), so the restore rule is the same rule at the same fidelity, applied to the channels warm-up never saw.
• It is transient. pendingBaseline is not part of DecoderState; it is derived at import and consumed by the next decode, so the schema version stays 4 and no checkpoint changes.
• calibrate() clears it, because a full calibration is a better answer than a pending one.
• The caveat, named: a channel that happens to be firing above rest at that first decode gets a high baseline and is under-rated afterwards. Warm-up avoids this by settling first, which a restore cannot do — the network is mid-run. A population rate is already an average over a window, so one sample is a reasonable estimate of rest; if this ever bites, the fix is to re-calibrate rather than to widen the window, because a window would leave the channels competing on raw rate while it filled.
• Visible next time: GET /status reports decoder — calibrated, pending, and one row per channel with its role, baseline and last score. Diagnostic and additive; nothing reads it back.

Pinned by a_restored_group_with_no_baselines_scores_every_channel_at_rest_then_on_merit in both twins and by the restore golden, which compares the scores of every channel at every step of a restored run.

The readout is fixed

The decoder calibrates once and then applies fixed ratios, timings and thresholds. No channel gain adapts, no task state enters scoring, and nothing in this module learns. Habituation is bounded motor history, not reward learning. The macro group changes none of that: it scores macro_<type> rates by the same formula on the same timings, and the only thing that can make one macro win more often than another is the reward rule acting on the mushroom body upstream of it (rewards and learning). Learning lives in the network, on the plastic edges described in plasticity. Source: readout/decoder.ts module comment, and fly-plays-pokemon/docs/ratchet-sprint.md, "Controller".

Display approximation (flystage)

apps/stage's NAMED CIRCUITS panel draws a faint tick at each pulse channel's decision threshold (A/B at 1, Start/Select at 1.35 after boot, from the table above), computed by inverting this page's own score formula: rate = threshold * (baseline + 1) - 1. The page cannot use the real baseline this module calibrates with — FeedHeader (docs/feed-protocol.md) never carries it — so it substitutes a running median of the role's rate as a stand-in "resting level". This is a display-only approximation: it does not feed back into decode() or any other part of this module, and it can disagree with the real calibrated baseline (most visibly right after a restore, before the median has had time to settle). See apps/stage/src/lib/circuit-scale.ts (RunningMedian, thresholdRateHz) for the implementation.

Bar scale is a separate, unrelated approximation: each bar's fill is a fraction of its own adaptive running-max reference (CircuitScale in the same module), not a fraction of this module's baseline. A bar reading "60% full" says nothing about where that role sits relative to its decoder score — only the threshold tick does that, and only for the two groups listed above.